The invention relates to optimization of the xerographic process, and more particularly, to the automatic compensation for toner concentration drift due to developer aging.
One benchmark in the suitable development of a latent electrostatic image on a photoreceptor by toner particles is the correct toner concentration in the developer. An incorrect concentration, i.e. too much toner concentration, can result in too much background in the developed image. That is, the white background of an image becoming gray. On the other hand, too little toner concentration can result in deletions or lack of toner coverage of the image.
Under prior art process controls, a relatively small toner control patch is developed and sensed to adjust the development process to maintain the quality of developed small solid areas.
Specifically, many machines (both copiers and printers) use optical feed back from toner patches to control DMA (developed mass per unit area). The toner patch is developed to a partially discharged region of a photoreceptor. In toner patch based DMA control systems, the patch voltage is held constant. The controller attempts to keep the reflectance of the toner patch in range using the toner dispenser as an actuator. When the toner patch reflectance is high (the patch is too light) toner is added. The assumption is that if toner is added in such a way that the toner patch reflectance is kept at its target value then the DMA of the printed foreground will be kept at its target. Toner patches are developed to partially discharged belt areas because patches developed to fully charged areas would be saturated black and have insufficient sensitivity to DMA to control toner concentration TC. To create a toner patch on a printer a small region of the photoreceptor is initially left unexposed (fully charged). A special discharge lamp is then used to reduce its surface potential to a target value a fixed number of volts above the developer bias. Toner is then developed to the patch and its reflectance is read by the optical sensor. As the toner patch gets developed toner is deposited on it until the development field is sufficiently neutralized. With highly charged toner, less toner will be developed to the patch and its reflectance will be below target causing toner to be added. With lower charged toner the opposite occurs.
The characteristic of some developer materials degrade over time. As the developer ages, its charging properties change and progressively lower toner concentrations are required to keep the toner patch reflectance at target. With some developer, the toner concentration gets set sufficiently low after as little as 30,000 prints that foreground solids can not be properly rendered. In this case sufficient toner is available to keep the low density toner patch at target but not enough toner is available to render the more demanding foreground solid areas.
An example of the prior art is, U.S. Pat. No. 4,999,673, assigned to the same assignee as the present invention, disclosing the use of a relatively small developed half tone image patch to regulate the developer parameters. However, these prior art small patch process controls are generally inadequate and insensitive to detect large solid area development deterioration as discussed above. It would be desirable, therefore, to provide a process control technique to detect deterioration in large, solid area development. It is also known in the prior art to use an electro-optic sensor or any other suitable sensor in the developer housing to determine toner concentration. The use of a sensor in the housing in addition to the IRD sensor normally used in adjusting development, however, adds additional cost and complexity to the system. It would also be desirable, therefore, to minimize additional cost and complexity in a developer control system that is capable of responding to large solid area development deterioration to maintain toner concentration and developed mass at a constant level throughout the life of the developer.
It is an object of the present invention therefore to provide a new an improved technique to detect deterioration in large, solid area development. It is another object of the present invention to minimize additional cost and complexity in a developer control system that is capable of responding to large solid area development deterioration to maintain toner concentration and developed mass at a constant level. Other advantages of the present invention will become apparent as the following description proceeds, and the features characterizing the invention will be pointed out with particularity in the claims annexed to and forming a part of this specification.